Ground fault relays protect electrical equipment from ground faults. A ground fault is an unintentional current path between a current-carrying conductor and a grounded surface. When a ground fault occurs, electric current may find a path to ground via dust, water, or worn insulation. Most short circuits in electrical equipment are caused by ground faults, which can also endanger worker safety. Some ground fault relays are designed to work with ungrounded systems. Products for motor protection may provide both predictive and protective features. Typically, output signals are sent to a remote meter or programmable logic controller (PLC).

Ground fault relays with on-line and off-line modes are designed to provide continuous protection from ground faults. Under normal conditions, these devices are used with a separately-connected current transformer. Both the alarm range and the time delay are adjustable. Metered loop connections and LED indicators are also available. If the load is switched-off, an auxiliary electrical contact causes the ground fault relay to change state. The small DC current that is imposed travels through the network. In a motor protection application, this current moves from the motor starter to the motor windings. Ground fault relays with an off-line feature may also have an active or failsafe mode.  

Ground fault relays carry product specifications such as nominal insulation voltage, insulation ground, test voltage, supply voltage, power input, on-line current relay, and response range. Products with alarm relay contacts differ in terms of switching capacity, rated contact voltage, continuous current, and breaking capacity. Linked terminals may be used for relay selection. Operating ambient temperature, storage ambient temperature, and mounting style are also important parameters to consider. Ground faults relays with terminal screws differ in terms of terminal capacity, weight, and enclosure dimensions.

Additional product specifications for ground fault relays include measuring voltage, measuring current, DC internal resistance, AC impedance at 50 - 60 Hz, response adjustment pot, maximum leakage capacitance to ground, maximum permissible DC stray voltage, and current transformer type. Ground fault relays for current transformers have parameters such as primary current, basic impulse level (BIL), rated frequency, thermal current rating. As a rule, a device’s BIL rating is a basic indicator of dielectric strength.